Belt of Ilmater (D&D 3.5e / Pathfinder 1e / D&D 5e / Pathfinder 2e)

Belt of Ilmater - D&D 3.5e (Original source)

Base: Monk's Belt (d20srd link)

• Armor Class is treated as a monk of five (5) levels higher. (At time of writing, Asher is level 12, which affords a +2 bonus to AC. With the belt, this becomes +3 as if he were level 17.)
  
• Unarmed damage is treated as a monk of five (5) levels higher. (AToW, the damage is 2d6. With the belt, the damage becomes 2d8.)
  
• One additional Stunning Fist attack per day.
  

Additional:

• Gain Dodge feat. If the Dodge feat was already taken by the wearer, the wearer can then dodge a second attacker as a free action.
  
• +2 to Hit and Damage with unarmed attacks.
  

Next evolution occurs at character level 13.

Pathfinder 1e does not have a Monk's Belt as a magic item. However, it does have Monk's Robe, which is a decent enough replacement, and we could then make this the Robe of Ilmater for this system.

The base abilities are (in accordance to the Pathfinder SRD):

• Armor class is treated as a monk of five levels higher (+3 at 12th level normally, +4 with the robe as if 17th level).
  
• Unarmed damage treated as a monk of five levels higher (2d6 at 12th level, 2d8 with the robe).
  
• One additional Stunning Fist attack per day.
  

Artifact effects at this level:

• Gain Dodge feat. If the Dodge feat was already taken by the wearer, the wearer can then dodge a second attacker as a free action.
  
• +2 to Hit and Damage with unarmed attacks.
  

Pretty straight forward, nothing really changed.

Right, this is the first snag I hit with any of the conversions.
D&D 5e does not have anything that's the equivalent to a Monk's Belt or Monk's Robe that I can use as the base. 
Therein lies the problem here.
Now it's easy to add in the additional material, but the base item itself is tough...
What I ended up doing was seeing how the base character turned out without the belt, and compared both the original 3.5e build and 5e build, and went from there, since some of the mechanics would be different.

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Now, in looking at how the Monk in 5e is built out... unlike in D&D3.5e and Pathfinder 1e, they don't get increasing bonuses to their Unarmored Defense as they gain levels. So a 20th level monk could have the same base Armor Class as a first level monk, unless they put all of their ability bonuses into DEX and/or WIS. I don't think that's quite fair, and could be potentially broken. I know there could be a vast difference between starting with, potentially, AC 15 at first level and ending up at AC 20 at 20th level (with everything capped and no magic items)... but how would that compare with a 20th level fighter with plate mail? (I'll have to work that one out, perhaps leave it to the philosophers.)

So, to counter that, and give the Monk's Belt in 5e a little bit of oomph, I could give it a flat +2 bonus to AC, or perhaps something that's a little more level-based, say +1 AC for every five levels, round up. (So, +1 AC at levels 1-5, +2 at levels 6-10, etc.)
I like the second, as it doesn't sound like so much of a game-changer.


Now let's look at the damage bonus. 
5e starts with a d4 and runs up to a d10 starting at 17th level. If we took the "damage die of a monk at five levels higher" theory, this breaks, as a 5th level monk has the same damage level (1d6) as a 10th level monk. Same with 11th and 16th levels with 1d8. Granted, the die type increases at 11th and 17th levels, so hopefully it's not that long of a wait. But why should one wait?
My rule: The Monk's Belt grants the wearer an increase in damage by one die step, so a strike that does 1d4 base damage now does 1d6, 1d6 would do 1d8, and so on. At 17th level, the 1d10 base damage becomes 1d12 and caps there. 

Stunning Strike... 
5e has this ability as well, but you need to spend one ki point to attempt it, rather than have so many attempts each day. This ability starts at 5th level. 
Now, since the Ki pool starts at 2nd level, so by 5th level your average monk would have 5 ki points, so one could potentially attempt this five times before a short or long rest is needed. This could be anywhere from five to fifteen attempts per day, assuming two short and one long rest per adventuring day.
What would be our options then? Additional ki only for a Stunning Strike? A bonus to the save DC of the strike?
If the former, I would consider the bonus to be per long rest only. 
This is why it's a tough call... I'd rather the latter, since there's the potential for plenty of attempts in a day. 
My rule: The wearer gets a bonus to the DC of a Stunning Strike equal to the user's Proficiency Bonus. (Even then, that's a deal-breaker... at 20th level, the base save DC is already 19, this would make the save DC for a Stunning Strike 25.)

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Now, the Dodge thing...
D&D 5e does not have a Dodge feat. It has a Dodge action, which means you focus on avoiding attacks, with no offensive actions. This puts a crimp on how this part of the Belt's ability would work.
The Dodge action basically imposes a penalty to hit in that all attacks against you are at a disadvantage, while any DEX-based saving throws are at an advantage. One could, in the words of Steve Rogers, "do this all day." And, apparently, this would affect any/all attacks against the person for that round. 
So... I'm thinking this...
- The Belt would grant the ability for the wearer to gain the Dodge benefit versus one opponent, and the effect lasts until the wearer's next turn. The wearer can invoke this ability a number of times equal to his Proficiency Bonus per short rest.
-- The Belt will allow the same effect versus a second opponent in a round. The wearer can invoke this ability a number of times equal to his Proficiency Bonus per long rest. (Yes, a Long Rest when it comes to dodging two opponents. at the same time.)

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So here's the base item:

Monk's Belt

(requires attunement)
This simple rope belt, when wrapped around a character’s waist, confers great ability in unarmed combat. 
The Monk's Belt grants the wearer an increase in damage by one die step, so a strike that does 1d4 base damage now does 1d6, 1d6 would do 1d8, and so on. At 17th level, the 1d10 base damage becomes 1d12 and caps there.
The Monk's Belt confers an Armor Class bonus of +1 for every five levels in Monk (round up), as long as the wearer does not wear any armor or use a shield.
The Monk's Belt grants the wearer a bonus to the DC of a Stunning Strike equal to the user's Proficiency Bonus.

Additional abilities:
The Belt of Ilmater grants a +2 bonus to hit and damage with a monk's Unarmed Strikes.
The Belt would grant the ability for the wearer to gain the Dodge benefit versus one opponent, and the effect lasts until the wearer's next turn. The wearer can invoke this ability a number of times equal to his Proficiency Bonus per short rest. The Belt will allow the same effect versus a second opponent in a round. The wearer can invoke this ability a number of times equal to his Proficiency Bonus per long rest. (Yes, a Long Rest when it comes to dodging two opponents. at the same time.)

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